The present invention relates to a semiconductor laser, a light source unit, and a laser light irradiation device.
A nitride semiconductor laser has been developed as a pick-up light source for a high-density optical disc and the like, and is recently deployed in various fields such as a projector, lighting, processing equipment, and the like, which require even higher output power. Especially in the processing applications including a precise measurement instrument for sensing or analyzation, a printing or exposure device, and a 3D printer, with requirements for high speed and high definition, it is required to achieve high power while retaining high beam quality.
In an application that requires high beam quality, a semiconductor laser of the type called a single transverse mode laser having a narrow refractive index waveguide structure as a light containment structure in a horizontal direction is commonly used. This laser oscillates a laser beam in a basic mode alone by cutting off a transverse higher mode of light by narrowing the waveguide, and thus makes it possible to constantly obtain a single-peaked beam intensity distribution. On the other hand, because the waveguide has a narrow width, a beam spot size of the end face is reduced to increase a light density on an emission end face, resulting that an upper limit of the light output is mainly limited by optical breaking strength of the end face.
As a method of improving the light output of the semiconductor laser, it is known to increase the beam spot size by extending the waveguide width. However, by simply extending the waveguide width, not only the single-peaked basic mode but also higher-order modes are allowed, which may degrade the beam quality.
As a method of increasing the output while retaining the single-peaked beam shape, the tapered waveguide structure is known in which the waveguide width is extended only in the end face portion (see, Japanese Unexamined Patent Application Publication No. 2003-101139 and Japanese Translation of PCT International Application Publication No. 2005-524234). The tapered waveguide structure is provided with a wide waveguide that allows for a higher mode near an end face, a narrow waveguide that allows for a basic mode alone at the center of a laser element, and further a tapered waveguide that smoothly couples the narrow waveguide and the wide waveguide. With the tapered waveguide structure, it is possible to minimize a waveguide loss in the basic mode while allowing the narrow waveguide to function as a filter preventing propagation of the higher mode by appropriately designing the dimension of the tapered waveguide. This makes it possible to increase the beam spot size without increasing an operating current compared with a related-art single-mode laser.